In recent years, coordinate inputting devices based on electrostatic capacitance, which can be structured, from thin transparent materials without using movable parts, have been used broadly as inputting devices that are well-suited for use with display devices, such as liquid crystal display panels, for detecting input operations into display locations.
In conventional coordinate inputting devices, a large number of X side electrodes and Y side electrodes are formed into the shape of a matrix, intersecting on the front and back surfaces of an insulating substrate, where an operation position of an object to be detected on the insulating substrate can be detected through the use of a change in the electrostatic capacitance of the intersecting X side electrodes and Y side electrodes in the vicinity of a proximate object to be detected, such as a finger. Such a device is described by Japanese Unexamined Patent Application Publication No. 2005-337773, hereafter referred to as “Patent Reference 1.”
In a coordinate inputting device as described by Patent Reference 1, predetermined pulse voltage is applied by scanning to a large number of Y side electrodes. While the pulse voltage is applied to each individual Y side electrode, the voltages for each of the X side electrodes that intersect with a Y side electrode to which the pulse voltage is applied are detected. The position of operation of the object to be detected on the insulating substrate is detected from the change in the electrostatic capacitance between the X side electrode and the Y side electrode that intersect at the position that is proximate to the object to be detected when an object to be detected, such as a finger, is in proximity to the insulating substrate, from the X side electrode wherein the voltage has changed because of the change in electrostatic capacitance, and the position of the Y side electrode to which the pulse voltage was applied at that time.
However, in order to detect the object to be detected, such as a finger, from the change in the electrostatic capacitance, the pitches of the X side electrodes and the Y side electrodes must for example be no more than 3 mm in the X and Y directions. When the input operation surface is a large surface area, the number of X side electrodes and Y side electrodes for which the change in electrostatic capacitance is to be detected grows large in accordance with the increase in the input surface area, causing the scanning period for scanning the intersection positions between each of the electrodes to be long, making it impossible to detect the operation position in a short period of time, and making it impractical for an input operation surface larger than for example an A4 sheet.
Given this situation, there is a known coordinate inputting device 100, structured as illustrated in FIG. 8, as a coordinate inputting device for detecting operation positions of objects to be detected or input operation surfaces of sizes larger than an A4 sheet. This device is described by Japanese Unexamined Patent Application Publication No. H8-137607, hereafter referred to as “Patent Reference 2”.
In the coordinate inputting device 100, operation position detection electrodes E1, E2, E3, and E4, for applying constant alternating current voltages, are disposed on the four corners of an insulating substrate 101 having a uniform resistance layer formed on the surface thereof. When a finger contacts any given position on the insulating substrate 101, an electric current flows to the finger through the resistance of the surface of the insulating substrate 101 from each of the electrodes E. At this time, the electric current values i1, i2, i3, and i4, which flowed to the finger from the respective electrodes E1, E2, E3, and E4 will vary depending on the distances from the electrodes at the four corners to the position of contact of the finger, and thus when the line connecting the opposite pair of electrodes E1 and E3 is defined as the X axis, and the line connecting the pair of electrodes E2 and E4 is defined as the Y axis, then the coordinate (X, Y) of the operation position that is the position of contact of the finger can be calculated from the following:X=i1/(i1+i3), and Y=i4/(i2+i4).
In this way, in the first conventional coordinate inputting device disclosed by Patent Reference 1 wherein a large number of X side electrodes and Y side electrodes are disposed in the form of a matrix, when the input operation surface is large, not only is time required for detecting the operation position, but also a multiplexer capable of handling a commensurate number of lines must be used in order to scan the increased number of X side electrodes and Y side electrodes, and thus there is a problem in that the circuit structure is complex and large.
In the second of the conventional coordinate inputting devices 100 disclosed by Patent Reference 2, which is directed to solve this problem, the operation position can be detected using only four electrodes; however, when a constant alternating current voltage is applied to each of the electrodes, it is necessary to apply, onto the insulating substrate, a resistive film wherein the resistance value per-unit-length is uniform, and thus the circuit structure is complex and large.
Furthermore, there may be a large detection error in the operation position when detecting the operation positions over the entire large input operation surface when using only the detection values that are detected by the four electrodes E that are disposed on the four corners of the insulating substrate 101. In particular, because the operation position is detected by having an electric current flow to the finger, it is not possible to apply an alternating current voltage that would cause a large electric current at each of the electrodes, and it has not been possible to detect the operation positions with good accuracy, for a large input operation surface, using the coordinate inputting device 100 that calculates the operation positions using only a trickle electric current value going from each electrode.